Re: [PATCH 1/2] mm/migrate_device.c: Copy pte dirty bit to page

[Peter Xu <peterx@redhat.com>]

Hi, Alistair,

On Fri, Aug 12, 2022 at 03:22:30PM +1000, Alistair Popple wrote:
> migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that
> installs migration entries directly if it can lock the migrating page.
> When removing a dirty pte the dirty bit is supposed to be carried over
> to the underlying page to prevent it being lost.
> 
> Currently migrate_vma_*() can only be used for private anonymous
> mappings. That means loss of the dirty bit usually doesn't result in
> data loss because these pages are typically not file-backed. However
> pages may be backed by swap storage which can result in data loss if an
> attempt is made to migrate a dirty page that doesn't yet have the
> PageDirty flag set.
> 
> In this case migration will fail due to unexpected references but the
> dirty pte bit will be lost. If the page is subsequently reclaimed data
> won't be written back to swap storage as it is considered uptodate,
> resulting in data loss if the page is subsequently accessed.
> 
> Prevent this by copying the dirty bit to the page when removing the pte
> to match what try_to_migrate_one() does.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Reported-by: Peter Xu <peterx@redhat.com>

This line should be:

Reported-by: Huang Ying <ying.huang@intel.com>

Please also feel free to add:

Acked-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu